The present invention relates to a device for observing an image of a specimen by using an electron beam and an image observation method using the same, and more specifically, the present invention uses a magnetic field at an upstream side in an electron beam traveling direction from the specimen to form the specimen image even in magnetic fields generated in an objective lens.
Lenses used in electron beam devices include two types: one produces a lens effect by an electric field, and the other produces the similar effect by a magnetic field. In the case of electron microscopes available as commercial products at present, most of them are of the latter type, and are called electromagnetic lenses. Of the electromagnetic lenses which are incorporated in these electron beam devices, the objective lenses of a transmission electron microscope TEM and a scanning type electron microscope are important elements which determine the performance of the devices.
FIG. 1 schematically shows a configuration of an ordinary magnetic field type objective lens. Main components comprise a magnetic pole piece 1, a coil 3 and a magnetic path 4. An amount of spherical aberration which mainly restricts the performance of a lens (=spatial resolution) becomes larger in proportion to a focal length. Therefore, at the time of actual use, an exciting current of the coil 3 is made large to generate a magnetic field at a level of a saturation magnetic field of a material constituting a magnetic yoke 4 and the magnetic pole piece 1, and the lens is used under the condition of a short focal length. At that time, in order to reduce aberration of the lens at a downstream side (here, the upper side of the paper surface is set as an upstream side, and the lower side is set as the downstream side) from the objective lens, it is general to make the magnification of the image formed by the objective about 100 times. In order to realize this, a distance between the objective lens and an object surface needs to be made about the focal length, and therefore, a specimen 2 needs to be placed in a magnetic field between the opposed magnetic pole pieces 1. It is a post-specimen magnetic field of the magnetic fields formed between both the magnetic pole pieces that contributes to image formation, and the specimen is observed in a state in which it is immersed in the magnetic field.
Pre-specimen magnetic fields are used for forming a very small electron spot in a scanning transmission electron microscope (STEM) (reduction projection of a crossover image on a specimen), for forming a convergent electron beam at a large angle in convergent beam electron diffraction (CBED), for miniaturization of an analysis region in analysis methods such as electron dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS), and for implementing collimated illumination in a TEM. As one example of these conventional arts, a geometrical-optical system diagram in the case of forming collimated illumination is shown in FIG. 18 by clearly drawing a lens generated by a pre-specimen objective lens field as an independent pre-specimen field lens 10. In FIG. 18, electron beams which incident on a condenser lens 5 form a crossover spot 6 on a focal surface at a front side of the pre-specimen field lens 10 by the condenser lens 5. Under this condition, electron beams at a rear side (downstream in an electron beam traveling direction) from the pre-specimen field lens 10 acts to form a crossover spot on the position of infinity from the pre-specimen field lens 10, and therefore, become collimated illumination as a result. Making collimated illumination possible in a wide range is very advantageous from the aspect of high resolution observation from the viewpoint of causing more detailed information to contribute to image formation.
As is apparent from these using methods, it is technology status that the pre-specimen magnetic field remains to be considered as a part of the illumination optical system located at the upstream side of the specimen in the traveling direction of the electron beams.
Such using methods of the electromagnetic lenses have the above described advantage, but have the following problems.
(1) A TEM has spatial resolution of an atomic order, and therefore, spatial measurement with high accuracy of a subnano meter order is possible by observation of a crystal lattice image. However, in an intermediate magnification region of about several tens thousand to a hundred thousand times, reproduction of the electronic optical system is insufficient due to hysteresis of the magnetic lens, and there is no standard specimens which are suitable for calibration of the magnification range and produced in volume at low cost. Therefore, with the specimen of which spatial size is not known, size measurement accuracy is insufficient.
(2) Further, while the magnifying power of the objective lens becomes high, and the magnifying power of several tens to a million times become easy as all the electron microscopes, realization of the magnifying power of the transitional range (×200 to 2000) between an optical microscope and an electron microscope becomes difficult. In order to realize the above described condition, there is no other measure than to suppress the magnifying power of the lens at the downstream side from the objective lens in the traveling direction of an electron beam, or to use lenses in combination so as to become a reduction system (lens power <1), and use in the state of large influence of image distortion and aberration is forced. Further, such conditions are significantly different in the use conditions of the lens from the above described high magnification observation. Therefore, it is often difficult to match the axes of all the lens with one another, and a special technique is required for adjustment of the electron optical system at the time of observation.
(3) Further, the specimen is placed in the magnetic field, and therefore, influence on the specimen by the magnetic field is not avoided when the specimen is a magnetic substance. In order to prevent the influence, there have been conventionally adopted the measures in which an image is formed with the lens at the downstream side from the objective lens without using the objective lens. The method does not require modification of the device side, and therefore, has been used for observation of a magnetic substance by a TEM for a long time, but with an extremely long focus and reduction in power of the lens which carries out image formation, reduction in the spatial resolution of the final observation image cannot be avoided (M. E. Hale, H. W. Fuller, and H. Rubinstein,; Journal of Applied Physics, vol. 30, p 789, 1959).
Further, there are proposed the measure in which the specimen position is moved to a distant position at the upstream side in the electron beam traveling direction from the magnetic yoke of the objective lens, and a magnetic shield is provided around the specimen (JP-A-06-283128). JP-A-06-283128 is accompanied by addition of a port for inserting a new specimen holding device to the device body, but is effective in not only the aspect of reducing the influence of the magnetic field received by the specimen, but also in the aspect of suppressing the magnifying power of the objective lens described in the previous paragraph by the movement of the specimen position. However, JP-A-06-283128 does not clearly describe the concrete moving amount of the specimen position and change in magnification accompanying the movement. Other than this, the techniques of JP-A-2005-32588 and “T. Hirayama, Q. Ru, T. Tanji, A. Tonomura,; Applied Physics Letters, vol. 63, p 418, 1993” in which a magnetic pole piece in a special shape which reduces the magnetic field around the specimen is incorporated can provide the effect of receiving no influence of the magnetic fields. However, in each of these countermeasures, a magnetic pole piece in a special shape corresponding to an observation target needs to be newly produced and replaced with the conventional magnetic pole piece.